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TOPIC: Black Holes


L

Posts: 131433
Date:
GRS1915+105
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Hurricane Katrina had nothing on GRS1915. This black hole seems to be rotating at least 950 times per second, hundreds of thousands of times faster than Katrina. The measurement marks the first time astronomers have been able to quantify the spin of a black hole so directly, and the findings could bring physicists closer to verifying the source of elusive cosmic phenomena known as gamma-ray bursts.

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L

Posts: 131433
Date:
GRS 1915+105
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A black hole has been found to be spinning faster than ever seen before, a new analysis suggests. The finding supports the idea that only fast-spinning stars can collapse to create powerful explosions called long gamma-ray bursts.
To measure the spin of black holes, astronomers measure the size of the discs of matter that orbit them. The faster a black hole spins, the closer matter can stably orbit around it. Watch an animation showing the difference between spinning and non-spinning black holes.
But the innermost edge of this disc is too small to see directly. So previous measurements of black hole spins have had to make assumptions about properties such as the tilt of the disc to Earth's line of sight.
Now, astronomers have measured the spin of a black hole with a new method that requires fewer assumptions. The team was led by Jeffrey McClintock of the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, US.
McClintock's team examined a black hole in our galaxy called GRS 1915+105, which lies about 36,000 light years away. Matter gets hotter as it gets closer to the black hole, so the team used X-ray observations from NASA's Rossi X-ray Timing Explorer to measure the temperature of the gas in the disc.
They found the innermost stable orbit around GRS 1915 is so close that the black hole must be spinning at nearly 1000 times per second – the fastest ever recorded.

But a second study of GRS 1915 suggests that the spin could be lower, according to an analysis of the same RXTE data by Matthew Middleton of the University of Durham, UK, and his colleagues.

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L

Posts: 131433
Date:
MCG-6-30-15
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Title: Suzaku observations of the hard X-ray variability of MCG-6-30-15: the effects of strong gravity around a Kerr black hole
Authors: Giovanni Miniutti, Andrew C. Fabian, Naohisa Anabuki, Jamie Crummy, Yasushi Fukazawa, Luigi Gallo, Yo****o Haba, Kiyoshi Hayashida, Steve Holt, Hideyo Kunieda, Josefin Larsson, Alex Markowitz, Chiho Matsumoto, Masanori Ohno, James N. Reeves, Tadayuki Takahashi, Yasuo Tanaka, Yuichi Terashima, Ken'ichi Torii, Yoshihiro Ueda, Masayoshi Ushio, Shin Watanabe, Makoto Yamauchi, Tahir Yaqoob

Suzaku has, for the first time, enabled the hard X-ray variability of the Seyfert 1 galaxy MCG-6-30-15 to be measured. The variability in the 14-45 keV band, which is dominated by a strong reflection hump, is quenched relative to that at a few keV. This directly demonstrates that the whole reflection spectrum is much less variable than the power-law continuum. The broadband spectral variability can be decomposed into two components - a highly variable power-law and constant reflection - as previously inferred from other observations in the 2-10 keV band. The strong reflection and high iron abundance give rise to a strong broad iron line, which requires the inner disc radius to be at about 2 gravitational radii. Our results are consistent with the predictions of the light bending model which invokes the very strong gravitational effects expected very close to a rapidly spinning black hole.

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L

Posts: 131433
Date:
Black Hole Census
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Astronomers using ESA's orbiting gamma-ray observatory, Integral, have taken an important step towards estimating how many black holes there are in the Universe.
An international team, lead by Eugene Churazov and Rashid Sunyaev, Space Research Institute, Moscow, and involving scientists from all groups of the Integral consortium used the Earth as a giant shield to watch the number of tell-tale gamma rays from the distant Universe dwindle to zero, as our planet blocked their view.

"Point Integral anywhere in space and it will measure gamma rays" - Pietro Ubertini, INAF, Italy and Principal Investigator on Integral's gamma-ray imager.

Most of those gamma rays do not come from nearby sources but from celestial objects so far away that they cannot yet be distinguished as individual sources. This distant gamma-ray emission creates a perpetual glow that bathes the Universe.

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Title: INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth
Authors: E. Churazov, R. Sunyaev, M. Revnivtsev, S. Sazonov, S. Molkov, S. Grebenev, C. Winkler, A. Parmar, A. Bazzano, M. Falanga, A. Gros, F. Lebrun, L. Natalucci, P. Ubertini, J.-P. Roques, L. Bouchet, E. Jourdain, J. Knoedlseder, R. Diehl, C. Budtz-Jorgensen, S. Brandt, N. Lund, N. J. Westergaard, A. Neronov, M. Turler, M. Chernyakova, R. Walter, N. Produit, N. Mowlavi, J. M. Mas-Hesse, A. Domingo, N. Gehrels, E. Kuulkers, P. Kretschmar, M. Schmidt

We study the spectrum of the cosmic X-ray background (CXB) in energy range ~5-100 keV. Early in 2006 the INTEGRAL observatory performed a series of four 30ksec observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background(CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalisation is ~ 10% higher. The CXB spectrum obtained by INTEGRAL agrees well with the measurements of RXTE/PCA, the latest recalculation of HEAO-1 measurements and ASCA and CHANDRA observations. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CBX origin.

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L

Posts: 131433
Date:
Supermassive black holes
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Supermassive black holes in some giant galaxies create such a hostile environment, they shut down the formation of new stars, according to NASA Galaxy Evolution Explorer findings published in the August 24 issue of Nature.

The orbiting observatory surveyed more than 800 nearby elliptical galaxies of various sizes. An intriguing pattern emerged: the more massive, or bigger, the galaxy, the less likely it was to have young stars. Because bigger galaxies are known to have bigger black holes, astronomers believe the black holes are responsible for the lack of youthful stars.

"Supermassive black holes in these giant galaxies create unfriendly places for stars to form. If you want to find lots of young stars, look to the smaller galaxies" - Dr. Sukyoung K. Yi of Yonsei University in Seoul, Korea, who led the research team.

Previously, scientists had predicted that black holes might have dire consequences for star birth, but they didn't have the tools necessary to test the theory. The Galaxy Evolution Explorer, launched in 2003, is well-suited for this research. It is extremely sensitive to the ultraviolet radiation emitted by even low numbers of young stars.
Black holes are monstrous heaps of dense matter at the centres of galaxies. Over time, a black hole and its host galaxy will grow in size, but not always at the same rate.
Yi and his collaborators found evidence that the black holes in elliptical galaxies bulk up to a critical mass before putting a stop to star formation. In other words, once a black hole reaches a certain size relative to its host galaxy, its harsh effects become too great for new stars to form. According to this "feedback" theory, the growth of a black hole slows the development of not only stars but of its entire galaxy.
How does a black hole do this? There are two possibilities. First, jets being blasted out of black holes could blow potential star-making fuel, or gas, out of the galaxy centre, where stars tend to arise.
The second theory relates to the fact that black holes drag surrounding gas onto them, which heats the gas. The gas becomes so hot that it can no longer clump together and collapse into stars.

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L

Posts: 131433
Date:
Star-forming QSO
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Title: Star-forming QSO host galaxies
Authors: P.D. Barthel

The recent finding of substantial masses of cold molecular gas as well as young stellar populations in the host galaxies of quasars is at odds with results of Hubble Space Telescope imaging studies, since the latter appear to yield mature, quiescent early type hosts. It is demonstrated here that the characterisation as 'quiescent' is incorrect.
Radio and far-infrared properties of both the HST sample and a larger comparison sample of uv-excess selected radio-quiet QSOs are consistent with substantial recent star-formation activity.

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L

Posts: 131433
Date:
Super-massive black holes
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European and American scientists, on a quest to find super-massive black holes hiding in nearby galaxies, have found surprisingly few. Either the black holes are better hidden than scientists realised or they are lurking only in the more distant universe.

Scientists are convinced that some super-massive black holes must be hiding behind thick clouds of dust. These dusty shrouds allow only the highest energy X-rays to shine through. Once in space, the X-rays combine into a cosmic background of X-rays that permeates the whole of space.
The search for hidden black holes is part of the first census of the highest-energy part of the X-ray sky. Led by Loredana Bassani, IASF, Italy, a team of astronomers published results in The Astrophysical Journal Letters in January this year. They show the fraction of hidden black holes in the nearby Universe to be around 15 percent, using data from ESA’s orbiting gamma-ray observation, the International Gamma Ray Astrophysics Laboratory (Integral).
Now astronomers from NASA Goddard Space Flight Center in Greenbelt, Maryland, and the Integral Science Data Centre near Geneva, Switzerland, have found an even smaller fraction using nearly two years of continuous data, also from Integral. The work shows that there is clearly too few hidden black holes in the nearby Universe to create the observed X-ray background radiation.

"Naturally, it is difficult to find something we know is hiding well and which has eluded detection so far. Integral is a telescope that should see nearby hidden black holes, but we have come up short" - Volker Beckmann of NASA Goddard and the University of Maryland, Baltimore County, lead author of the new report to be published in an upcoming issue of The Astrophysical Journal.

The X-ray sky is thousands to millions of times more energetic than the visible sky familiar to our eyes. Much of the X-ray activity is thought to come from black holes violently sucking in gas from their surroundings.
Recent breakthroughs in X-ray astronomy, including a thorough black hole census taken by NASA's Chandra X-ray Observatory and Rossi X-ray Timing Explorer, have all dealt with lower-energy X-rays. The energy range is roughly 2 000 to 20 000 electron-volts (optical light, in comparison, is about 2 electron-volts). The two Integral surveys are the first glimpse into the largely unexplored higher-energy, or 'hard', X-ray regime of 20 000 to 300 000 electron-volts.

"The X-ray background, this pervasive blanket of X-ray light we see everywhere in the universe, peaks at about 30 000 electron volts, yet we really know next to nothing about what produces this radiation" - Neil Gehrels of NASA Goddard, a co-author.

The theory is that hidden black holes, which scientists call Compton-thick objects, are responsible for the 30 000 electron-volts peak of X-rays in the cosmic X-ray background. Integral is the first satellite sensitive enough to search for them in the local universe.
According to Beckmann, of all the black hole galaxies that Integral detected less than 10 percent were the heavily shrouded 'Compton thick' variety. That has serious implications for explaining where the X-rays in the cosmic X-ray background come from.

"The hidden black holes we have found so far can contribute only a few percent of the power to the cosmic X-ray background" - Volker Beckmann.

This implies that if hidden black holes make up the bulk of the X-ray background, they must be located much further away in the more distant universe. Why would this be? One reason could be that in the local universe most super-massive black holes have had time to eat or blow away all the gas and dust that once enshrouded them, leaving them revealed.
This would make them less able to produce X-rays because it is the heating of the gas falling into the black hole that generates the X-rays, not the hole itself. So, if the black hole had cleared its surroundings of matter there would be nothing left to produce X-rays.
Conversely, another possibility is that perhaps the hidden black holes are more hidden than astronomers realised.

"The fact that we do not see them does not necessarily mean that they are not there, just that we don’t see them. Perhaps they are more deeply hidden than we think and so are therefore below even Integral's detection limit" - Volker Beckmann .

Meanwhile, the NASA team is now planning to extend his search for hidden black holes further out into the universe.

"This is just the tip of the iceberg. In a few more months we will have a larger survey completed with the Swift mission. Our goal is to push this kind of observation deeper and deeper into the universe to see black hole activity at early epochs. That’s the next great challenge for X-ray and gamma-ray astronomers" - Volker Beckmann .

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L

Posts: 131433
Date:
Black Hole Growth
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Black holes in the early universe may have circumvented a law of physics to grow rapidly to colossal size. The finding could solve a longstanding puzzle over why such massive objects appeared so soon after the universe began.

The new analysis, by Marta Volonteri and Martin Rees, both at the University of Cambridge, UK, ties up all the important factors involved in the growth of a black hole and concludes rapid growth is possible. This might be because the black hole "swallows" the radiation generated as the hole gobbles up the matter around it, preventing a destructive explosion.

The puzzle first arose after astronomers spotted what appear to be monster black holes, with the mass of a billion Suns, near the edge of the visible universe. The black holes themselves are invisible, but matter falling into them is heated by friction and emits very powerful X-rays. These extreme emissions define the distant system as a quasar.
Because of the time it takes for the X-rays to travel from these extremely distant objects to Earth, astronomers see the quasars the way they were less than a billion years after the big bang.
All consuming
Until now, astronomers could not explain how the objects gathered such enormous amounts of matter in such a relatively brief time. One suggestion was that black holes in the early universe somehow overcame a law called the "Eddington limit", which normally restricts the growth of objects that are collecting matter.
The limit arises because if a black hole eats too quickly, the disc of matter feeding it radiates so much energy that it blows itself apart, leaving little for the object to absorb, so halting growth.
Some astronomers have suggested that early black holes managed to get around this law by swallowing the radiation in its vicinity before it had a chance to blow apart the disc of matter.
In the dense inner part of the disc, the X-rays might have a hard time travelling outward because of its frequent collisions with matter. If so, it could get pulled into the black hole along with the descending matter.
Eats, shoots out and leaves

But it was not clear whether even these "outlaw" black holes would grow fast enough, given other constraints that exist. For example, black holes can occasionally get kicked out of matter-rich clouds into intergalactic space, where there is nothing to eat. However, the analysis by Volonteri and Rees shows the growth can be fast enough.

"This growth is quite a challenge, the challenge can be met under special, but plausible, circumstances" - Stuart Shapiro, of the University of Illinois at Urbana-Champaign in the US.

Those circumstances include an assumption that the rate at which black holes get kicked out of host galaxies is relatively low.
Eventually, the black holes would go back to eating at less than the Eddington limit due to a lack of supply

"Most holes in present-day galaxies are inconspicuous because they are starved of fuel." - Martin Rees

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L

Posts: 131433
Date:
Black Holes
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Most of the biggest black holes in the universe have been eating cosmic meals behind closed doors – until now.

With its sharp infrared eyes, the Spitzer Space Telescope (SST) peered through walls of galactic dust to uncover what may be the long-sought missing population of hungry black holes known as quasars.

"From past studies using X-rays, we expected there were a lot of hidden quasars, but we couldn't find them. We had to wait for Spitzer to find an entire population of these dust-obscured objects" - Alejo Martínez-Sansigre of the University of Oxford, England. He is lead author of a paper about the research in this week's Nature.

Quasars are super-massive black holes that are circled by a giant ring of gas and dust. They live at the heart of distant galaxies and can annually consume up to the equivalent mass of one thousand stars. As their black holes suck in material from their dusty rings, the material lights up brilliantly, making quasars the brightest objects in the universe. This bright light comes in many forms, including X-rays, visible and infrared light.

Astronomers have puzzled for years over the question of how many of these cosmic behemoths are out there. One standard method for estimating the number is to measure the cosmic X-ray background. Quasars outshine everything else in the universe in X-rays. By counting the background buzz of X-rays, it is possible to predict the approximate total number of quasars.

But this estimate has not matched previous X-ray and visible-light observations of actual quasars, which number far fewer than expected. Astronomers thought this might be because most quasars are blocked from our view by gas and dust.

They proposed that some quasars are positioned in such a way their dusty rings hide their light, while others are buried in dust-drenched galaxies. Spitzer appears to have found both types of missing quasars by looking in infrared light. Unlike X-rays and visible light, infrared light can travel through gas and dust.

Researchers found 21 examples of these quasars in a small patch of sky. All the objects were confirmed as quasars by the National Radio Astronomy Observatory's Very Large Array radio telescope in New Mexico and by the Particle Physics and Astronomy Research Council's William Herschel Telescope in Spain.

"If you extrapolate our 21 quasars out to the rest of the sky, you get a whole lot of quasars. This means that, as suspected, most super-massive black hole growth is hidden by dust." - Dr. Mark Lacy of the Spitzer Science Center, California Institute of Technology (Caltech), Pasadena, California, a co-author of the Nature paper.

The discovery will allow astronomers to put together a more complete picture of how and where quasars form in our universe. Of the 21 quasars uncovered by Spitzer, 10 are believed to be inside fairly mature, giant elliptical galaxies. The rest are thought to be encased in thick, dusty galaxies that are still forming stars.


Position (2000): RA: 17h14m29.67s Dec: +59d32m33.5 s

This false-colour image from NASA's Spitzer Space Telescope shows a distant galaxy (yellow), 3.5 billion light years away in the constellation Draco, that houses a quasar, a super-massive black hole circled by a ring, or torus, of gas and dust. Spitzer's infrared eyes cut through the dust to find this hidden object, which appears to be a member of the long-sought population of missing quasars. The green and blue splotches are galaxies that do not hold quasars.
Astronomers had predicted that most quasars are blocked from our view by their tori, or by surrounding dust-drenched galaxies, making them difficult to find. Because infrared light can travel through gas and dust, Spitzer was able to detect enough of these objects to show that there is most likely a large population of obscured quasars.
In addition to the quasar-bearing galaxy shown here, Spitzer discovered 20 others in a small patch of sky. Astronomers identified the quasars with the help of radio data from the National Radio Astronomy Observatory's Very Large Array radio telescope in New Mexico. While normal galaxies do not produce strong radio waves, many galaxies with quasars appear bright when viewed with radio telescopes.
In this image, infrared data from Spitzer is coloured both blue (3.6 microns) and green (24 microns), and radio data from the Very Large Array telescope is coloured red. The quasar-bearing galaxy stands out in yellow because it emits both infrared and radio light.
Of the 21 quasars uncovered by Spitzer, astronomers believe that 10 are hidden by their dusty tori, while the rest are altogether buried in dusty galaxies. The quasar inside the galaxy pictured here is of the type that is obscured by its torus.

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